This invention relates to post-rinse compositions and processes for improving the corrosion protective qualities of conversion coatings on metal substrates. Such materials and processes may alternatively be called, among other names, "final rinses", "passivating rinses or treatments!", "post-treatments", or "sealing rinses or treatments!". Their fundamental characteristics are that (1) they are contacted with the substrate to be treated only after completion of formation of a conversion coating and therefore normally have no direct contact with the underlying metal substrate, except possibly through pores in the conversion coating and (2) contact with the post-rinses, optionally followed by application of one or more additional protective coatings, results in a surface with better corrosion resistance, in at least one type of service or accelerated test procedure, than that shown in the same service or test by otherwise identical substrates treated in the same way, except for substituting contact with pure water for contact with the post-rinse.
Before the development of modern environmental consciousness in the last few decades, aqueous solutions containing hexavalent chromium were almost always used as post-rinses after the most common kinds of conversion coatings, those in which the predominant anions are phosphate and/or chromate ions. Hexavalent chromium is now regarded as a serious pollutant, however, so that eliminating or at least minimizing its use has been an objective of the metal finishing industry for about twenty years. Even so, no fully satisfactory replacement for aqueous hexavalent chromium containing solutions for all post-rinse uses has yet been provided.
In addition to causing pollution problems, hexavalent chromium containing aqueous solution post-rinses must normally, in order to obtain the maximum degree of corrosion resistance increase available from using them, be dried into place on a substrate treated with them before any subsequent operation in which the substrate previously treated with a hexavalent chromium containing post-rinse is contacted with another aqueous liquid composition. Even after such drying into place of a hexavalent chromium containing aqueous solution post-rinse, chromium can be leached from a substrate treated with it by contact with certain kinds of liquids subsequently used to treat the same substrate. This can cause paint discoloration and similar undesirable blemishes on the treated surface and can substantially reduce the corrosion protective value of the hexavalent chromium containing post-rinse aqueous solution treatment.
It is believed that the most generally satisfactory replacements for post-rinses containing hexavalent chromium that are now widely known and used in the art are aqueous solutions/dispersions containing predominantly, among their active ingredients, base polymers of vinyl phenol, on at least some of the aromatic rings of which have been grafted alkyl and/or hydroxy alkyl amino methylene groups, substituting for hydrogen atoms that would be present on the aromatic rings in a simple polymer of vinyl phenol. Syntheses and uses of these polymers are described in the following U. S. Patents to Lindert et al., all of which, except to any extent that may be inconsistent with any explicit statement herein, are hereby incorporated herein by reference: U.S. Pat. No. 4,433,015 of Feb. 21, 1981; U.S. Pat. No. 4,517,028 of May 14, 1985; U.S. Pat. No. 4,963,596 of Oct. 16, 1990; U.S. Pat. No. 5,039,770 of Aug. 13, 1991; and U.S. Pat. No. 5,063,089 of Nov. 5,1991. These materials, which may be collectively denoted hereinafter as "amino modified polyvinylphenol", have been found technically satisfactory in most types of post-rinse applications but have a substantially higher cost than conventional hexavalent chromium containing post-rinses. This higher cost has hampered acceptance of the amino modified polyvinylphenol type post-rinses.
Accordingly, one object of this invention is to provide a more economical, but still chromium free, post-rinse composition and process compared with previously used amino modified polyvinylphenol post-rinses. Another alternative or concurrent object is to provide a post-rinse composition and process after use of which the treated surface may be freely rinsed with aqueous liquid compositions without substantially reducing the corrosion protective quality that otherwise could be achieved. Other concurrent or alternative objects will be apparent from the description below.
Except in the claims and the operating examples, or where otherwise expressly indicated, all numerical quantities used in the description of the invention to indicate amounts of material or conditions of reaction and/or use are to be understood as modified by the word "about" in describing the broadest scope of the invention. Practice within the numerical limits stated is generally preferred, however. Also, throughout the description, unless expressly stated to the contrary: percent, "parts of", and ratio values are by weight; the term "polymer" includes "oligomer", "copolymer", "terpolymer", and the like; the description of a group or class of materials as suitable or preferred for a given purpose in connection with the invention implies that mixtures of any two or more of the members of the group or class are equally suitable or preferred; description of constituents in chemical terms refers to the constituents at the time of addition to any combination specified in the description or of generation in situ within the composition by chemical reaction(s) noted in the specification between one or more newly added constituents and one or more constituents already present in the composition when the other constituents are added, and does not necessarily preclude unspecified chemical interactions among the constituents of a mixture once mixed; specification of materials in ionic form implies the presence of sufficient counterions to produce electrical neutrality for the composition as a whole and for any substance added to the composition; any counterions thus implicitly specified preferably are selected from among other constituents explicitly specified in ionic form, to the extent possible; otherwise such counterions may be freely selected, except for avoiding counterions that act adversely to an object of the invention; the word "mole" means "gram mole", and the word itself and all of its grammatical variations may by used for any chemical species defined by all of the types and numbers of atoms present in it, irrespective of whether the species is ionic, neutral, unstable, hypothetical, or in fact a stable neutral substance with well defined molecules; and the terms "solution", "soluble", "homogeneous", and the like are to be understood as including not only true equilibrium solutions or homogeneity but also dispersions that show no visually detectable tendency toward phase separation over a period of observation of at least 100, or preferably at least 1000, hours during which the material is mechanically undisturbed and the temperature of the material is maintained within the range of 18.degree.-25.degree. C.